As a few have mentioned Bolter & Chainsword have a donation system on their site, and then once a year there is a drive through thread and post to reach the required level for the next years expect funding.
Personally my hobby focus, time and budget is often spent on one thing for several months and then I take a change and do something different for a few months. So a donation system is my preferred method of supporting a forum as I will donate whatever I can spare when that forum is part of my main focus at the time.
So for example last year I did a lot of 30k and 40k stuff with a break in the middle so I spent a lot of time on B&C and therefore donated, this year so far I have been focusing on AoS so have spent more time here and next to none on B&C, therefore if I had the option I would be happy to donate to support this forum at the moment.
I would support a subscription based system as long as using the main forum and features are still free to use so that when I can contribute I take out the subscription for a period of time but when I have nothing to spare or my interests lie elsewhere then I would let it come to an end and not renew it until I had the interests and means again.
As others have said please no adds. I can ignore one or two banner adds easily enough, and do so for forums related to some other hobbies I have from time to time, but it always feels like the begging of a slippery slope aside from the other issues with adds.